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In a symmetric-key system, Bob knows Alice's encryption key. Once the message is encrypted, Alice can safely transmit it to Bob (assuming no one else knows the key). In order to read Alice's message, Bob must decrypt the ciphertext using which is known as the decryption cipher, :
Pad C n with the extracted ciphertext in the tail end of D n (placed there in step 3 of the ECB encryption process). P n = Head (D n, M). Select the first M bits of D n to create P n. As described in step 3 of the ECB encryption process, the first M bits of D n contain P n. We queue this last (possibly partial) block for eventual output. P n− ...
The key is generated from the message in some automated fashion, sometimes by selecting certain letters from the text or, more commonly, by adding a short primer key to the front of the message. There are two forms of autokey cipher: key-autokey and text-autokey ciphers.
With the Caesar cipher, encrypting a text multiple times provides no additional security. This is because two encryptions of, say, shift A and shift B, will be equivalent to a single encryption with shift A + B. In mathematical terms, the set of encryption operations under each possible key forms a group under composition. [26]
Despite its problems, the one-time-pad retains some practical interest. In some hypothetical espionage situations, the one-time pad might be useful because encryption and decryption can be computed by hand with only pencil and paper. Nearly all other high quality ciphers are entirely impractical without computers.
In cryptography, the EFF DES cracker (nicknamed "Deep Crack") is a machine built by the Electronic Frontier Foundation (EFF) in 1998, to perform a brute force search of the Data Encryption Standard (DES) cipher's key space – that is, to decrypt an encrypted message by trying every possible key.
To decrypt, the procedure is simply reversed. Longer messages are first broken up into blocks of fixed length, called the period, and the above encryption procedure is applied to each block. One way to detect the period uses bigram statistics on ciphertext letters separated by half the period.
The Rail Fence cipher is a form of transposition cipher that gets its name from the way in which it is encoded. In the rail fence cipher, the plaintext is written downward and diagonally on successive "rails" of an imaginary fence, then moves up when it gets to the bottom.